Although there are a number of different types of thermal cut-off switches, many of them are overly complex and utilize more parts than are commercially feasible for many applications. The construction of a simple, inexpensive, but effective thermal cut-off device using a minimum number of parts, therefore, continues to be a challenge. One attempt at providing a simplified thermal cut-off switch design utilizing a minimum number of parts is shown in Japanese Utility Model Laid-Open Publication No. 15922/72, which was filed under the name of Murata Manufacturing Company Limited. In this type of device, a hollow structure having a normally elongated oblong shape with a continuous circumference is connected directly to the insulated lead.
When the pellet is inserted into the device and the oblong contact is forced against the pellet, it is bowed outwardly until it contacts the outer metallic housing of the device. The Murata device, however, can be difficult to manufacture due to the small size of the parts and because of the necessity of either providing either a hole in the contact through which the head of the insulated lead may pass, or of some other means of permanently securing the hollow contact to the head of the insulated lead which will not be affected by high temperatures. The contact of the Murata device engages the thermal pellet at essentially a single point, thereby subjecting the pellet to a high stress which tends to destroy the integrity of the organic pellet and to reduce the reliability of the device because of premature opening of the switch.
Another design which utilizes a bowed hollow contact member with a continuous circumference or perimeter is shown in U.S. Pat. No. 4,167,724, issued Sept. 11, 1979, in the name of James R. McCaughna. The McCaughna device differs from the Japanese Murata device in that the hollow contact structure of this switch has a general rectangular shape and it is not permanently secured to the head of the insulated lead. Instead, the insulator through which the insulated lead passes has a reduced diameter portion which is encircled by a coiled spring which tends to force the contact member toward the thermally-sensitive pellet. The McCaughna thermal cut-off switch, unlike the Murata switch, has a flat contact area which engages the thermal pellet; and thus, there is a better force distribution on the thermal pellet in the McCaughna switch. However, even with this improved force disbribution, there is still a potential problem of overstressing the pellet and a premature opening of the switch. Part of this stress problem can be relieved by inserting a wafer between the hollow contact member and the organic pellet; but even with the wafer, a premature opening of the McCaughna switch may occur at a higher rate than desirable.
The thermal cut-off switch of the present invention may employ essentially the same elements as the McCaughna thermal cut-off switch, with the exception that the hollow generally rectangular-shaped, bowed contact member is replaced by a contact which does not have a continuous circumference or perimeter, but instead has a partially open top with inwardly directed surfaces that make contact with the head of the insulated lead at a point which is in close proximity to the point where the coiled spring engages the contact member. By use of this contact member, a more reliable switch is provided due to a release of excessive force on the pellet, while at the same time maintaining good electrical contact with both the housing and the insulated lead. Alternate constructions of the contact member are formed as a thin, split-ring configuration or as a cup-shaped member with an inwardly bent segmented rim so that the contact area is formed around the entire outer surface of the contact member and the entire head of the insulated lead. In the latter contact member, the sides are slotted in order to allow the melted organic material to flow through them, and thus this contact resembles a plurality of split-rings joined at the bottom.